The invention relates to vapor generation apparatus generally and more specifically to a tube stop structure used to transmit loading due to seismic disturbances or strong winds that act on such apparatus. Seismic disturbances produce a horizontal load that is a function of the weight of the boiler.
Typical furnace constructions have horizontal buckstays disposed in bands around the perimeter of the furnace tube walls at vertically spaced intervals (often between 15 and 40 feet) throughout the height of the furnace wall. Tube stops are typically disposed between two or three of the horizontal buckstays and the tube wall. Unlike the stirrup connections between the spiral tube wall and the horizontal buckstay the tube stop must transfer a shear load.
While the invention has particular application to furnaces having a spiral tube wall construction. The spiral wall furnace design increases the mass flow per tube by reducing the number of tubes needed to envelop the furnace without increasing the spacing between the tubes. This is done by arranging the tubes at an angle and spiraling them around the furnace. For instance, the number of tubes required to cover the furnace wall can be reduced to one-half by putting the tubes at a 30 degree angle. The angle chosen for the particular application ordinarily will vary between 7 and 30 degrees. An advantage of the spiral wall construction is that every tube is part of all of the wall which means that each tube acts as a heat integrator around the four walls of the combustion chamber.
Conventional tube stop constructions heavy, thick weldments that are welded to the furnace tube walls. Such constructions cause thermal stresses that result in cracking and failure of the furnace tube wall.
Spiral tube wall furnace power boilers are capable of extremely quick changes in power output. These quick changes in power output result in extremely quick changes in the tube wall temperatures. Because of this, apparatus such as the tube stop in accordance with the invention must be designed with particular attention to thermal stress problems.
Another design consideration is field fit-up problems during final assembly. The furnaces to which the present invention has particular application are partially assembled in a shop and then final assembly is completed at a job site. Because of the relatively large size of the apparatus tolerance build-up problems are particularly acute.